The finding comes from a small study of just six people. To confirm the interaction, larger studies are needed. But the finding supports what doctors have been saying for years: Herbal remedies are strong medicine. They can interact with prescription drugs in unexpected ways, says researcher Wei C. Lau, MD, director of adult cardiac anesthesia at the University of Michigan Health System.
"Drugs can interact with drugs, and drugs can interact with herbals - and the result in either case can be dangerous," Lau says in a news release.
Plavix is a drug given to prevent blood clots. It's usually given to people who have had a heart attack or a stroke caused by a blood clot. It helps reduce the risk of future heart attacks or strokes by preventing blood clots. However, not everyone responds to the drug.
Lau's team studied six healthy volunteers. In earlier studies, these volunteers were found to be resistant to the blood-thinning effects of Plavix. The researchers say this is likely due to underactivity of a liver enzyme that normally activates Plavix.
St. John's wort increases the activity of this enzyme. So researchers put these patients on Plavix and St. John's wort.
The researchers say that by taking the herb, Plavix is activated in the body, allowing it to have a stronger blood-thinning effect.
Sure enough, the herb greatly increased Plavix's effect.
No one should take St. John's wort to increase the effects of Plavix. This study shows that the two together could cause dangerous blood thinning that could lead to severe bleeding - even in the brain.
But if this finding is confirmed in larger studies, it may help patients who appear to be resistant to the effects of Plavix get more out of the drug.
Meanwhile, the study serves as a reminder to tell your doctor about all the herbal remedies you may be taking.
Lau reported the study findings in a poster presentation at this week's Annual Scientific Sessions of the American College of Cardiology in Orlando, Fla.